The system for supporting a long structural member typically includes a pin joint type system in which a moment does not act on the supporting portion at each end of the structural member and a fixed joint type system in which a moment acts on the supporting portion. In the fixed joint type system a deflection angle at the end of the structure is generally zero, and in the pin joint type system it is never zero. These phenomena are observed both in an inner pipe of a double pipe structure consisting of a main pipe for sustaining axial force and a stiffening pipe encircling the main pipe and in an outer pipe of a double pipe structure consisting of a main pipe and a stiffening pipe encircled by the main pipe. An example of the double steel pipe consisting of an outer main pipe and an inner stiffening pipe is disclosed in JP1992-149345A1.
It is necessary that a double steel pipe structural member for restraining buckling thereof does not buckle to be stable under the axial compression. The Official Guide for Steel Structure Buckling Design regulates several conditions in order not to buckle a structural member, e.g. “Preventing the ends of a structural member from being damaged” in relation to the present invention, thus a reinforcing member of a mouth piece type or a core metal type mentioned after has been introduced into the structural member.
A structural member made of double steel pipe is described hereinafter by giving an example in which the structural member is applied as a diagonal brace to a grid of the framework consisting of columns and beams. The right and left columns of the framework are loaded with lateral forces under an earthquake to lean in any direction, then the upper beam moves in the lateral direction relative to the lower beam. The framework deforms alternately to a parallelogram and to a parallelogram of the reverse shape while axial compressive force and axial tensile force act on the brace by turns, the brace made of a double pipe is loaded through only the main pipe, but is not loaded through the stiffening pipe which is supported at only one point so as not to fall out of the structural member. It is necessary for the stiffening pipe to have the bending resistance properties for preventing the main pipe from buckling and as such, has to remain straight.
The axis at the end of a pin support type main pipe for sustaining axial force always intersects the axis of the stiffening pipe, which is clearly different from the non-intersectional configuration in a cruciform plate joint type pipe, shown in JP2007-186894A1, fixedly supported at its both ends. When the stiffening pipe is an outer pipe, the larger the main pipe deforms, the closer the end of the main pipe comes towards the inner surface of the stiffening pipe. If the clearance between the main pipe and the stiffening pipe is small, even a slight flexure of the main pipe makes the end of the main pipe contact with the inner surface of the stiffening pipe. When the main pipe deforms heavily, a reaction force from the stiffening pipe causes deformation of the end of the main pipe, or a compressive force from the end of the main pipe causes deformation of the stiffening pipe.
In order to introduce a double pipe into a framework by using a pin joint, a clevis joint is available as shown in JP2009-193639A1. Engaging each clevis with a mouth piece by a right hand helix and left hand helix allows the length of the main pipe, namely, the distance between the eyes of both clevises, to be minutely controllable in proportion as the distance between both pins specified in a framework. A suitable over engagement of the helices permits the main pipe to be desirably pre-stressed.
A steel pipe is applied to a stiffening pipe so as to easily restrain the main pipe for sustaining axial force from bending. But the main pipe is sometimes damaged at the end thereof before the stiffening effect generated by the stiffening pipe appears. In order to avoid the damages of the main pipe and the deformation of the structural member, a cylindrical reinforcing member is fixed to the end of the main pipe. When the stiffening pipe is used as an inner pipe, a core metal to be inserted into the opening of the end of the stiffening pipe is integrated with the counter-clevis side of the mouth piece fixed to the end of the main pipe.
In the case where the stiffening pipe is an outer pipe and the reinforcing member is fixed to the end of the inner pipe (see JP1996-68110A1), the clearance between the reinforcing pipe and the stiffening pipe has to be large enough so that the inner pipe with the reinforcing pipe can be inserted into the outer pipe. When the stiffening pipe is an inner pipe and the reinforcing member is fixed to the end of the outer pipe (see JP1994-93654A1), the clearance between the stiffening pipe and the core metal used as the reinforcing member has to be large enough so that the core metal can be inserted into the inner pipe.
If the clearance mentioned above is excessively large, the stiffening pipe cannot function as a bending resistance pipe while the main pipe does not contact with the stiffening pipe in spite of the fact that the main pipe has already bent. The longer the reinforcing member and the core metal are, the more buckling restriction effect is improved. However, over-length of the reinforcing pipe or the core metal results in increasing in the weight of the structure member, over-shortage of them results in decreasing in buckling restriction effect generated by the stiffening pipe.